1. Field of the Invention
The invention relates generally to the field of active source acoustic characterization of subsurface rock formations and passive seismic emission mapping of subsurface microseismic events. More particularly, the invention relates to characterization and fluid distribution mapping with respect to time of subsurface rock formations having fluid pumped therein.
2. Background Art
Passive seismic emission tomography is a process in which an array of seismic sensors is deployed in a selected pattern on the Earth's surface (or on the water bottom in marine surveys) and seismic energy is detected at the sensors that emanates from various seismic events occurring within the Earth's subsurface. Processing the signals detected by the sensors is used to determine, among other things, the position in the Earth's subsurface and the time at which the various seismic events took place.
Applications for passive seismic emission tomography include, for example, determining the point of origin of microearthquakes caused by movement along geologic faults (breaks in rock layers or formations), movement of fluid in subsurface reservoirs, and monitoring of movement of proppant-filled fluid injected into subsurface reservoirs to increase the effective wellbore radius of wellbores drilled through hydrocarbon-producing subsurface Earth formations (“fracturing”). The latter application, known as “frac monitoring” is intended to enable the wellbore operator to determine, with respect to time, the direction and velocity at which the proppant filled fluid moves through particular subsurface Earth formations.
Passive seismic emission tomography for the above types of interpretation includes determining what are subsurface originating seismic events from within the signals detected at each of the seismic sensors, and for each event detected at the seismic sensors, determining the spatial position and time of the origin of the seismic event. Passive seismic interpretation methods known in the art are undergoing continuous improvement to better resolve the source of seismic events originating from the Earth's subsurface. One particularly useful technique for passive seismic monitoring is described in U.S. Pat. No. 7,663,970 issued to Duncan et al.
Active or controlled source seismic evaluation of subsurface rock formations includes a technique described in International Patent Application Publication No. WO 2009/062286, the application for which was filed by Guigné et al. and is commonly owned with the present invention. The evaluation technique described in the WO '286 publication includes directing seismic energy, by repetitive operation of a seismic source such as a vibrator or air gun array, preferably along a selected direction or to a selected point in the subsurface, and detecting seismic energy from the source after its interaction with the formations by using an array of seismic sensors deployed above the subsurface formations. The sensitivity of the array may be beam steered to a selected position within the subsurface. Repetitive actuation of the source and stacking of the detected energy combined with beam steering the array response may result in higher frequency seismic energy being detected from the subsurface than is otherwise obtainable using conventional common mid point (CMP) reflection seismic survey techniques. Such higher frequency content may enhance the resolution of the seismic information obtained from the detected signals.
It is desirable to evaluate how the petrophysical properties of fractured subsurface formations change as a result of hydraulic fracturing in order that fluid flow properties of wellbores drilled through a subsurface can be optimized.